It has previously been recognized that molecules contained in a liquid sample can be stimulated using a suitable stimulating light source. Lasers are often used as the stimulating light source. Other electromagnetic beams may also be useful as stimulating beams. The stimulating beam is pulsed into the sample. The beam pulses cause volumetric changes in the sample which generate pressure waves. The resultant pressure waves are detected by a suitable acoustic transducer, such as a piezoelectric transducer. The transducer is acoustically s coupled to the sample container to enhance detection of the pressure waves. The resulting pressure wave information has been used in analytical chemistry for characterizing one or more components of the sample. The technique of light stimulation and associated acoustical detection is more commonly known as photoacoustic analysis.
U.S. Pat. No. 4,303,343 describes a photoacoustic system which uses a laser light or other stimulating beam. The stimulating beam is presented in pulses of duration described therein to be 10.sup.-7 to 10.sup.-2 seconds. The beam pulses are directed upon a liquid or bulk solid sample having low absorbance of the stimulating beam. The frequency of the stimulating beam is varied to provide data used to define a characterizing curve indicating energy absorbance as a function of the stimulating beam frequency. This information is used in characterization of the sample compounds.
Despite the prior systems for photoacoustic analysis, there remains a need for improved photoacoustic analytical methods having the ability to resolve rapid molecular conformational changes. Molecular conformational changes are of general interest for a broad range of chemicals. They are of particular interest in the analysis of peptides, proteins, and other biological molecules for which current methods of analysis are of only limited usefulness.
There is also a need for improved methods for chemical analysis of reactions, reaction intermediates and reaction mechanics during very short periods of time after a chemical stimulus has occurred. Prior art analytical systems provide "stop-action" analysis for chemical reactions by rapidly mixing two solutions in about 1 millisecond. The resulting mixture is monitored using absorbance or fluorescence detection methods.
Alternatively, it is possible to use calorimetric analytical methods to better understand chemical reaction processes. Calorimetric analytical techniques are employed due to the importance of energy relationships between reactants and products. The energy change associated with reactions are thus measurable using calorimetric techniques. For extremely fast reactions, it is sometimes difficult to measure enthalpic changes. The reactions must either be slowed by cooling, or fast calorimetric methodologies must be used.
Such systems do not allow resolution of the reaction process when there are very rapid transformation periods on the order of 10.sup.-10 to 10.sup.-8 seconds. Rapid transformation capability is sought so that the equivalent of stop action or freeze frame data can be obtained during periods of chemical degradation, reaction between two distinct reactants, molecular conformational changes, and other similar transformations herein referred to as reactions.
Molecular conformational changes of many molecules are believed occur within a time range of 10.sup.-10 to 10.sup.-5 seconds after a chemical stimulus for change has been presented. Prior techniques have not been effective at obtaining information which can be used to provide freeze frame information concerning the processes involved in conformational changes, such as in conformational changes demonstrated in biological molecules.